Error: 'nullptr' was not declared in this scope, because of header file of Tesseract

[tlimkim]

System information (version)

• OpenCV => 3.4.0
• Operating System / Platform => linux ubuntu 16.04
• Compiler => 5.4.0

Detailed description

Hello.

When I build the OpenCV, there is a compile error: ‘nullptr’ was not declared in this scope (see below for details). I think there is a problem with 'tesseract' that is support c++11.

If anyone who has an idea, please help this problem.

Thanks.

/usr/local/include/tesseract/tesscallback.h: In member function 'virtual R _ConstTessMemberResultCallback_1_5<del, R, T, P1, A1, A2, A3, A4, A5>::Run(A1, A2, A3, A4, A5)':
/usr/local/include/tesseract/tesscallback.h:8369:17: error: 'nullptr' was not declared in this scope
       member_ = nullptr;

[alalek]

Try OpenCV's ENABLE_CXX11=ON CMake flag.

[tlimkim]

Thanks for replying.

I set CMake flag but it didn't work.
Instead of doing that, I added a line of code that enable to compile c++11 in CMakeLists.txt

Steps to reproduce

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")

When I added this line and compile the OpenCV, there was no problem while compiling the headerfile of Tesseract.

Can I make a pull request about this issue?

[alalek]

I set CMake flag but it didn't work.

Did you see this CMake output (without your patch)?

--   C/C++:
--     Built as dynamic libs?:      YES
--     C++11:                       YES                  <---------------------
--     C++ Compiler:                /usr/bin/c++  (ver 5.4.0)

[tlimkim]

I saw that CMake output which is
C++11: YES
but it didn't affect compiling the OpenCV with Tesseract header files.

So I thought that there is a problem.. because there is correct for CMake output on C++11 but it doesn't work for actual compiling.

[alalek]

Run make VERBOSE=1 and check passed compiler options (for -std=c++11).

[alalek]

BTW, Did you have enable precompiled headers?

3.4.0

There was an issue with PCH processing (CMake doesn't provide native PCH support).
Check this commit: opencv/opencv@2aa18ca#diff-980653262620d7624fd21ad16af8796f (available in 3.4.1)

[tlimkim]

In my cmake PCH source code (forked from few days ago), there isn't the codes that above issue.

What is the difference between modify CMakeList.txt and above issue?